The present invention relates to an improvement of a cover screw for a dental implant to seal a threaded hole, which is open on the oral cavity side of an implant fixture implanted in an implantation hole formed in a mandible or maxilla (hereinafter sometimes referred to as only "mandible") of the deficit tooth portion, until the implanted implant fixture is thoroughly connected to the mandible in the implantation hole.
Hitherto, as the dental prosthetic method in a missing tooth portion, a method for using a bridge and a method for using a denture have been generally employed.
However, in the method to use a bridge, since healthy natural teeth on both sides of the missing tooth portion are cut to prepare abutment teeth with which a metallic body provided for a dental prosthesis to be positioned in the missing tooth position is engaged, the method had such defects that not only the healthy natural teeth must be cut to provide engaging portions, but also a bone resorption is likely caused in the missing tooth portion because an occlusion pressure is not directly applied to the dental prosthesis part positioned in the missing tooth portion.
Also, in the method to use a denture, an artificial tooth is fixed to a denture base made of a synthetic resin or the like to prepare a dental prosthesis. According to this method, the occlusal force which is applied to the dental prosthesis is supported by the remaining natural teeth and/or the oral mucosa adjacent to the denture. Therefore, the method involves such defects that a sense of incongruity is generated during the use of the dental prosthesis, and the denture base covers receptors of the taste in the oral mucosa tissue, thereby generating blunting of the taste. In addition, the method has a serious defect that a loss of the residual ridge is caused during the use for a long period of time.
Thus, as a therapeutic method for overcoming these defects, a technology of dental implant has been developed and used, in which an implant fixture acting as a retention of a dental prosthesis or an artificial tooth root is implanted in an implantation hole formed in a mandible of the missing tooth portion to be connected to the mandible to function as a tooth root in a natural tooth, and a fixture for a dental prosthesis is connected and fixed to the abutment fixed to the implant fixture in its oral cavity side, whereby a dental prosthesis is fixed to this fixture for a dental prosthesis.
When a therapeutic method to use such a dental implant is used, the dental prosthesis can be fixed without covering the oral mucosa. Therefore, this method has advantages such that neither a sense of incongruity nor blunting of the taste occurs when the dental prosthesis is installed, and the dental prosthesis is felt like natural teeth. Also, the bone resorption highly expected in the case where no implant fixture is implanted can be suppressed at the minimum because a proper occlusal force is imparted to the mandible. Thus, this therapeutic method has been developed rapidly to such an extent that it can be applied to not only the case where a single tooth is lost, or two or more teeth are lost but also the case of an edentulous jaw (i.e., all teeth are lost).
In the therapeutic method to use such a dental implant, a double technique is mainly employed in which, after an implant fixture implanted in an implantation hole formed in a mandible is thoroughly connected to the mandible of the missing tooth, and the operated portion where the implantation hole has been formed is healed, a gingiva portion at the oral cavity side having the implant fixture implanted therein is again cut to fix an abutment, in which a fixing device of a dental prosthesis is installed, to the oral cavity side of the implant fixture.
In the therapeutic method to use the dental implant according to the double technique, the first operation includes, 1) cutting and opening a gingiva portion at a missing tooth portion in which an implant fixture is implanted, to make a gingival flap, 2) forming an implantation hole for an implant fixture in an exposed alveolar bone of a mandible, 3) implanting the implant fixture within the implantation hole, 4) in order not only to seal a threaded hole which is open on the oral cavity side of the implanted implant fixture but also to prevent a bone of the mandible from growing more inward toward the oral cavity side, screwing a cover screw for a dental implant into the threaded hole which is open on the oral cavity side of the implanted implant fixture, 5) covering the exposed alveolar bone of the mandible with the gingival flap while returning the gingival flap to the original position, and 6) suturing the gingival flap at the cut and opened site. The cover screw has a truncated conical portion with an end portion on the implant fixture side having a size equal to or larger than that of an end portion of the column like portion of the implant fixture and the outer diameter increasing gradually in a direction apart from the implant fixture, and a succeeding truncated cone like portion with the outer diameter decreasing gradually in a direction apart from the implant fixture. The cover screw also includes a concave portion on the face on the implant fixture side, in which a rectangular pipe portion projecting from the end of the implant fixture on the oral cavity side can be inserted, and a screw at the center of the concave portion.
The second operation is carried out after the implant fixture implanted in the implantation hole formed at the mandible of the missing tooth portion is thoroughly connected to the mandible within the implantation hole (usually it takes from 3 to 6 months). The second operation includes, 1) cutting and opening the gingiva portion at the top of the alveolar bone in which the implant fixture is implanted, to form a gingival flap, 2) exposing the cover screw for the dental implant, 3) removing the cover screw for the dental implant, which is screwed in the implant fixture, 4) screwing a screw into the threaded hole of the implant fixture, 5) returning the gingiva petal to the original position and covering the exposed alveolar bone of the mandible with the gingival flap while the healing abutment is exposed, and 6) suturing the gingival flap at the cut and opened site. The screw is formed on the implant fixture side of a healing abutment, and is structured such that an end portion thereof on the implant fixture side has a size approximately equal to that of the end portion of the implant fixture on the oral cavity side, a length thereof is larger than a thickness of the surrounding gingiva, and the shape thereof on the implant fixture side is approximately the same as that of the abutment.
In the second operation, it takes about one month for the cut site to heal. When the gingiva touches the healing abutment on the implant fixture side, the healing abutment is removed. An abutment, a shape thereof on the implant fixture side being approximately identical to that of the healing abutment on the implant fixture side, is disposed at the oral cavity side of the implant fixture. An intermediate bolt is screwed into the threaded hole of the implant fixture while penetrating the abutment to fix the abutment to the implant fixture on the oral cavity side. Then, a gold cylinder having a dental prosthesis on its outside face is disposed on the oral cavity side of this abutment. The dental prosthesis is fixed to the implant fixture on the oral cavity side via the abutment by the intermediate bolt penetrating the gold cylinder or by screwing a gold screw into a threaded hole formed in the abutment, resulting in completing the operation for the dental implant.
When the first operation for the above mentioned dental implant is carried out, the implantation hole for the implant fixture is formed in the alveolar bone of the mandible which is exposed by cutting and opening the gingiva at the missing tooth portion where the implant fixture is implanted. When the implant fixture is implanted in the implantation hole, in case of an insufficient amount of bone in the implantation site of the mandible or of the bone defect, it happens that the implanted implant fixture is exposed to the oral cavity side. If there is a possibility that the undesirable event mentioned above occurs, it is desirable to cover the exposed portion of the implant fixture in order to ensure an area of the bone binding site of the implant fixture and to reduce a risk of a bone resorption caused by a load. Therefore, a bone graft or filling of a bone graft material is often performed before or at the same time of implanting the implant fixture.
However, the bone graft using the bone from a patient oneself increases the burden to the patient, and has a limitation for collecting a sufficient amount of bone. On the other hand, the bone graft using the bone from other person or animal has problems, such as infection and invasion of protein of other person or animal into a body, and the use of the bone graft material has a defect that it can not be replaced by a bone. In addition, there is sometimes a problem that it is hard to graft the bone satisfactorily or to fill the bone graft material into the site to fill.
In order to solve the above mentioned problems, it is ideal that an insufficient bone is supplemented in a body of a patient. Recently, a guided tissue regeneration method using a barrier membrane has been established and used in a dental clinical field. The guided tissue regeneration method provides a space for a regenerated bone under the periosteum of a bone defect portion using the barrier membrane, and induces a bone tissue there by filling blood clot. Therefore, it is important to fix the barrier membrane in order to provide a space.
The conventional method to fix the barrier membrane, used to perform the guided tissue regeneration method, includes;
1. a method in which the barrier membrane is disposed over a cover screw screwed to an implant fixture, and the barrier membrane is fixed to a mandible by driving an absorbable or non-absorbable pin into the mandible. PA1 2. a method in which the barrier membrane with a hole formed by removing a portion corresponding to a cover screw is disposed over a cover screw screwed to an implant fixture, and the barrier membrane is fixed to a mandible by driving an absorbable or non-absorbable pin into the mandible. PA1 3. a method in which the tissue barrier membrane is fixed by disposing it between an implant fixture and a cover screw when the cover screw is screwed to the implant fixture.
However, in the first method, it is very difficult to handle the pin in an oral cavity because the pin is very small. The operation to fix the barrier membrane is complicated because a plurality of pins are used, and the alveolar bone is invaded by the pin used. If the bone grows and touches the face of the cover screw on the oral cavity side, an amount of bone to be cut to detach the cover screw increases in the second operation in which the cover screw is detached and a male screw of a healing abutment is screwed to the threaded hole of the implant fixture, resulting in increasing a load to a patient.
On the other hand, in the second method, in case there is a bone defect portion around the oral cavity side of an implant fixture in an alveolar bone of a mandible in which an implantation hole for an implant fixture is formed in a missing tooth portion, there is a possibility that a portion of the fixed barrier membrane on the implant fixture side sinks in the bone deficit portion. Also, there is a possibility that an undesirable periodontal tissue comes out from a boundary between a portion of the implant fixture on the oral cavity side and the hole of the barrier membrane.
In the third method, in case of using a non-absorbable membrane as the barrier membrane, there is a possibility that the mechanical surface property of the surface of the implant fixture on the oral cavity side becomes worse when the barrier membrane is attached or removed. Therefore, during the second operation, it may happen that the healing abutment can not be fixed closely to the implant fixture, or when the healing abutment is removed and a dental prosthesis is fixed, the abutment can not be fixed closely to the implant fixture. In case of using an absorbable membrane, as the barrier membrane is absorbed, an undesirable periodontal tissue may invade between the surface of the implant fixture on the oral cavity side and the cover screw. When the invasion happens, there is a possibility that during the removal of the periodontal tissue, the mechanical surface property of the surface of the implant fixture on the oral cavity side becomes worse, resulting in such events that during the second operation, the healing abutment can not be fixed closely to the implant fixture, or when the healing abutment is removed and a dental prosthesis is fixed, the abutment can not be fixed closely to the implant fixture. There is, also, a serious defect that the bone tissue may not possibly grow enough to reach the side surface of the cover screw.
The object of the present invention is to solve the defects of the conventional method mentioned above, and to provide a device for securely fixing in an ideal condition a barrier membrane, which provides a space for a regenerated bone under the periosteum by a guided tissue regeneration method using a barrier membrane when an implant fixture implanted in an implantation hole formed in a mandible at the missing tooth portion is connected to the mandible and/or maxilla within the implantation hole.